This invention relates generally to an exposure method. More particularly, the invention is concerned with an exposure method wherein a first exposure process as can be represented by a standard or ordinary exposure process, such as projection exposure, and a second exposure process of a higher resolving power than the first exposure process, are performed so that different types of patterns are printed superposedly to thereby form a desired pattern (hereinafter xe2x80x9cgoal patternxe2x80x9d) of a smallest linewidth corresponding to the second exposure process. The present invention is suitably applicable to the manufacture of chips or devices such as semiconductor chips (e.g., ICs or LSIs), detecting devices (e.g., magnetic heads), micro-machines or image pickup devices (e.g., CCDs), for example.
Currently, many projection exposure apparatuses for the manufacture of devices such as ICs, LSIs or liquid crystal panels, for example, based on photolithography, use a light source of an excimer laser. However, mere use of such an excimer laser as a light source in a projection exposure apparatus does not assure production of a fine pattern of a linewidth of 0.15 micron or narrower.
In order to improve the resolution, theoretically, the numerical aperture (NA) of a projection optical system may be enlarged or the wavelength of exposure light may be shortened. Practically, however, enlargement of the NA or reduction in wavelength of the exposure light is not very easy. For, the depth of focus of a projection optical system is inversely proportional to the square of the NA while it is proportional to the wavelength xcex. Because of this, enlarging the NA of a projection optical system results in a decrease of the depth of focus, which causes a difficulty of focusing and a decrease of productivity. Further, most glass materials have an extraordinarily low transmission factor with respect to a deep ultraviolet region. Even for a fused silica (quartz) which is used with a wavelength xcex=248 nm (KrF excimer laser), the transmission factor reduces almost to zero when used with a wavelength xcex=193 nm or less. Currently, no glass material has been developed that can be practically used in a region of exposure wavelength xcex=150 nm or less, corresponding to a fine pattern of 0.15 micron linewidth or less to be produced in accordance with a standard or ordinary exposure process.
Japanese Patent Application, Application No. 304232/1997, now Laid-Open No. 11-143085 (hereinafter, xe2x80x9cthe earlier Japanese patent applicationxe2x80x9d), filed by the assignee of the subject application, proposes a dual exposure process which is based on a combination of dual-beam interference exposure and standard exposure, wherein a multiple-value exposure amount distribution is applied to a substrate, to be exposed, to assure high resolution exposure. In an embodiment disclosed in the earlier Japanese patent application, the dual-beam interference exposure process is performed by use of a phase shift mask having a line-and-space (LandS) pattern of 0.1 micron linewidth, and a fine linear pattern is printed through coherent illumination. Thereafter, an ordinary exposure process (for example, an exposure process based on partially coherent illumination) is performed while using a mask which is formed with a pattern having portions of different transmission factors and having a shape corresponding to an actual device pattern of a smallest linewidth of 0.1 micron. In accordance with the method disclosed in the earlier Japanese patent application, a pattern of a smallest linewidth of 0.10 micron may be formed through the ordinary exposure process and by using a projection exposure apparatus having a projection optical system which has an image side NA of 0.6.
Another method for the fine pattern printing is a probe exposure method wherein a pattern is drawn and printed on a photosensitive member by using a probe. The probe may be based on AFM using an inter-atomic force, STM using a tunnel current, an electron beam, a laser beam or proximity light, for example. However, performing the probe exposure over the whole exposure area has a disadvantage of low throughput. In consideration of it, those portions of a goal pattern that can be produced through an ordinary exposure process may be photoprinted by using a light quantity larger than an exposure threshold of a photosensitive substrate. On the other hand, those portions of insufficient resolution may be photoprinted by superposed printing which is based on an ordinary exposure and a probe exposure, with the respective light quantities each being lower than the exposure threshold of the photosensitive material but both, when combined, being higher than the exposure threshold. As a result, a multiple-value exposure amount distribution similar to that described above is applied (Japanese Patent Application, Application No. 137476/1998).
It is an object of the present invention to provide an improved exposure method by which a multiple exposure process such as described above can be applied to assure higher precision production of a device such as a semiconductor chip, for example.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.